The present invention relates to a slip structure of a timepiece used for a center wheel or the like of the timepiece.
In an analog timepiece, rotation of a drive source such as a step motor is transmitted to hands such as an hour hand, a minute hand, and the like through a gear train mechanism to move the hands, thereby indicating a time. A center wheel has a slip structure so that a second wheel on which the second hand is fitted and the step motor are not rotated when the positions of the minute and hour hands are adjusted during, e.g., time correction.
More specifically, the center wheel integrally comprises a gear portion and a shaft portion having a canon pinion. During a normal timepiece operation, the gear and shaft portions are integrally rotated to move the minute hand. During adjustment of the positions of the minute and hour hands, e.g., time correction, when a torque larger than a predetermined value is applied to the canon pinion of the shaft portion, only the shaft portion is rotated to rotate the minute and hour hands, while the second wheel and the step motor are not rotated.
In the conventional slip structure described above, the gear and shaft portions of the center wheel are made of a metal, and a pair of leaf spring-like elastic segments are provided to the metal gear portion to extend in parallel in an insertion hole of the gear portion, to which the shaft portion is inserted, so that the canon pinion of the shaft portion is elastically clamped by the pair of elastic segments. For this reason, the number of parts is large, assembly is complicated, and a stable slip torque cannot be obtained resulting in cumbersome machining and high cost.
In order to eliminate these drawbacks, Japanese Utility Model Publication No. 55-29829 discloses that shaft portion 31 and gear portion 32 of a center wheel are formed of a synthetic resin by an injection melding, and are combined to be capable of slipping with each other, as shown in FIG. 1.
In order to prepare the center wheel described above, at first, gear portion 32 is molded. Then, molded gear portion 32 is placed in metal molds for molding shaft portion. Thereafter, a resin is injected into the metal molds to form shaft portion 31. Since shaft portion 31 shrinks upon cooling, gear portion 32 is clamped by extended portions 31a of shaft portion 31, thereby combining shaft and gear portions 31 and 32 to be capable of slipping.
With this structure, the slip torque of the center wheel is determined by the clamping force and the frictional resistance of extended portions 31a. In order to reduce the slip torque, the clamping force and the frictional resistance need only be reduced. For this purpose, extended portions 31a can be reduced in size in the radial direction. However, if extended portions 31a are reduced in size, a joint strength between shaft and gear portions 31 and 32 is weakened. As a result, shaft and gear portions 31 and 32 may be disengaged from each other during assembly. Therefore, a decrease in fitting depth is limited, and hence, it is difficult to reduce the slip torque. Therefore, the above-mentioned structure cannot be adopted in the slip structure of a small timepiece such as a wristwatch.
Further, during molding of the shaft portion, the gear portion which has already molded may be distorted due to a temperature of the forming resin for the shaft portion, so that the slip torque may become unstable.